Briefly explain the formation of dimmers and trimmers.

Some compounds like hydrogen fluorides, ethanoic acids, benzoic acid form dimmers or trimmers by joining up of their two or three molecules by H-bonding. Sometime, due to the formation of these dimmers or trimmers the molecular weight of these compounds appears to be doubled or tripled. That is, ethanoic acid has molecular weight of 60, but in a dimmer of this molecule the molecular weight appears to be 120.

The value of surface tension and viscosity of water also increase due to H-bond formations. Increase in surface tension of water allows a steel blade or a leaf to float on water. Glycerin is viscous due to formation of H-bond in its molecules.
In removal of the water of crystallization of some salts, like CuSO4, the last molecule of water is lost by application of high temperature. A copper sulphate molecule has five molecules of water by crystallization. Of these, four molecules are attached to the Cu2+ by coordinate bond and the fifth water molecule is attached to the SO2-4 ion by H-bond. The four molecules of water attached to the Cu2+ ion are lost at 100°C turning the crystal white, but the fifth molecule is not lost unless the temperatures rises to 230°C. After losing all water molecules copper sulphate turns into a white powder.


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